1. Field of the Invention
The present invention relates generally to photodetectors for sensing and measuring radiant energy, and particularly visible, invisible and near light wavelength energy, and more particularly to a photodetector including an electrically conductive window which is transparent to the desired light and near light wavelengths while precluding interference in such desired measurements by other electromagnetic radiation such as radio frequency interference, power line emissions and other spurious sources.
2. Description of the Prior Art
Photodetectors are generally available in the form of devices that sense illumination falling thereon. Measurements may be qualitative, i.e., the presence or absence of illumination, or quantitative, i.e., a measurement of the intensity of illumination thereon. Generally photodetectors take the form of a sensitive area which varies as to electrical properties as a function of light or related radiation striking the sensitive area. Typical of such known devices are photovaristors (commonly cadmium sulphide and lead telluride devices), phototransistors (typically junction transistors with the base exposed to a light source in order to modulate the collector current as a function of the light intensity striking the base), photodiodes (solid state diode devices in which PN junctions are sensitive to light incident thereon to alter current), photoresistor materials, photoconductive materials, photovoltaic cells, etc. In general the solid state devices such as phototransistors and photodiodes, and particularly photodiodes, are considered the more sensitive of the photodetector devices.
Photodetectors are often employed in very demanding and sensitive measurement processes. Typical of the demanding measurement processes involved are the non-invasive oximeter devices disclosed in Shaw U.S. Pat. No. 3,638,640 and Konishi et al U.S. Pat. No. 3,998,550. In such environments, the photodetector is placed adjacent a thin section of living tissue in order to measure the light passing through the tissue and, as a function of the light measured, determine the oxygenation of blood within such tissue. Clearly such determination may be, in some instances, a life and death measurement. The subjects of such determination may be patients exposed to a number of sources of interfering radiation. Surgical patients, for example, are often monitored by a number of devices, i.e., electrocardiograms, electroencephalograms, and other body function measuring electronic devices, as well as being subject to radio frequency currents utilized in electrosurgery. Accordingly, the patient's body is a significant source of static electricity and various electromagnetic radiation including radio frequency interference and 60 Hz AC emissions. Such radiation is fully capable of registering spurious signals in photodetectors, and particularly solid state photodetectors. By rectifying such radiation, the photosensitive material, in the case of solid state photodetectors, may produce a current. Also, conductance of the photodetector may be induced by a "field effect" caused by such interference. For these reasons, a significant problem exists with conventional photodetectors utilized in an enviroment in which spurious signals may be induced as a result of these various electromagnetic energy sources other than the desired radiant energy, which is typically visible, invisible and near light frequency energy.